Historical Articles

A.S.T.M. Designation: B252-51
T, Issued 1951**

*Under the standardization procedure
of the American Society for Testing Materials, this recommended practice is
under the juris diction of A. S. T. M. Committee B-8 on Electrodeposited Metallic
Coatings.
**Accepted by the American Society for Testing Materials, at Annual Meeting,
June, 1951.
***1949 Book of A. S. T. M. Standards, Part 2.

This Tentative Recommended Practice
has been approved by the sponsoring committee and accepted by the Society in
accordance with established procedures, for use pending adoption as standard.
Suggestions for revisions should be addressed to the Society at 1916 Race Street,
Philadelphia 3, Pa.

SCOPE
1. This recommended practice is intended as an aid in establishing and maintaining
a preparation cycle for electroplating zinc-base die castings conforming to
the Standard Specifications for Zinc-Base Alloy Die Castings (A.S.T.M. Designation:
B 86)***. It may be used in the production of the electrodeposited coatings
conforming to the Tentative Specifications for Electrodeposited Coatings of
Nickel and Chromium on Zinc and Zinc-Base Alloys (A.S.T.M. Designation: B 142)3.
This recommended practice is advisory only and is not a part of either specification.

NATURE OF ZINC-BASE DIE CASTINGS
2. (a) The alloys used in the manufacture of zinc-base die castings are made
with special high grade zinc conforming to the Standard Specifications for Slab
Zinc (Spelter) (A.S.T.M. Designation: B 6) 3 alloyed with about 4 per cent of
aluminum, 0.04 per cent of magnesium, and, depending upon the particular alloy
involved, 0, 1, or 3 per cent of copper. Impurities such as lead, cadmium, and
tin are held rigorously at specified low levels.
(b) Die castings made of these alloys are characterized by dense, fine-grained
and, usually but not always, smooth surfaces. Complex shapes frequently are
involved. In general, zinc surfaces are more sensitive than steel to contact
with alkali, and care must taken to avoid prolonged contact with strongly alkaline
cleaners if blistering of copper primary coatings is to be avoided. When a die
lubricant is needed in the die casting process, the proper choice will minimize
part of the problem of cleaning.

POLISHING PARTING LINES
4. (a) This operation may be carried out on set-up wheels or abrasive belts.
To obtain the most rapid cutting, the coarsest abrasive size should be used
that will permit the desired final smoothness to be obtained in the subsequent
buffing operation. Sizes in the range No. 180 to No. 220 mesh commonly are used.
The abrasive surface may be lubricated, with a suitable substance, with a resulting
finer texture.
(b) Linear speeds of 6000 to 8000 surface feet per minute (1890-2400 m/min)
should not be exceeded on set-up wheels. For abrasive belts, linear speeds of
5000 to 7000 surface feet per minute (1500-2100 m/min) should not be exceeded.
Lower speeds, of the order of 3500 to 4000 surface feet per minute (1100-1200
m/min) are used in many plants. The following equations, relating revolutions
per minute and wheel diameter to linear speed, may be helpful:

POLISHING OTHER SURFACES
5. While many die castings are sufficiently smooth to require only buffing,
spot or over-all polishing may be necessary in some instances. A No. 220-mesh
abrasive is generally used on set-up wheels or abrasive belts although finer
sizes may suffice at times. Linear speeds of 6000 to 8000 surface feet per minute
(1800 - 2400 m/min) should not be exceeded on set-up wheels or 5000 to 7000
surface feet per minute (1500-2100 m/min) on abrasive belts. Slower speeds,
of the order of 3500 to 4000 surface feet per minute (1100-1200 m/min), are
used in many plants. The abrasive surface should be lubricated.

BUFFING
6. (a) Buffing of zinc-base die castings should be carried out on cloth wheels
of suitable stiffness at linear speeds not exceeding 7000 to 9000 surface feet
per minute (2100-2700 m/min), slower speeds, of the order of 3500 to 5000 surface
feet per minute (1100-1500 m/min), are used in many plants. There is available
a good selection of compounds from which to choose. After buffing, or after
buff and color, it is common practice to clean the surface by passing it over
a relatively clean, dry buffing wheel. In some instances, a separate coloring
operation may be needed. The polishing, buffing, and coloring steps must be
properly graduated or else a poor surface, or the necessity for an additional
operation, may result. The buffing compound should be made with a binder which
is readily emulsified or saponified during the cleaning operation.

(b) Care in the buffing operation
will be repaid by a considerable simplification of the ensuing cleaning cycle.
The packing of hardened compound into holes and recesses adds greatly to the
cleaning problem. Extra care in avoiding such packing (in some cases wiping
to remove excess compound may be considered desirable) will reduce the demand
placed on the precleaning operation.

PRECLEANING
7. (a) The preliminary removal, preferably as soon after polishing and buffing
as possible, of most of the grease and soil in a precleaning operation is recommended
strongly. The uneven distribution of soil on the average surface results in
early local grease removal and prolonged alkali contact in the electrocleaner
while heavier films elsewhere on the surface are being removed.

(b) The complexity and cost of the
precleaning cycle will depend upon the type, distribution, and character of
the soil to be removed. Under best conditions, there will be present a thin,
reasonably uniform film of grease, which is readily removed in a simple sequence
of degreasing and electrocleaning. Under worst conditions, hard-caked, sometimes
partially carbonized buffing compound, will be packed into holes and recesses,
removal of which may require several precIeaning steps, including some hand
scrubbing. It is impossible to describe a single practice applicable in all
cases.

(c) There are several ways by which
greasy soil can be removed from zinc-base die castings prior to alkaline cleaning.
Generally speaking, these fall into three main classes, solvent degreasing,
emulsion cleaning, and cleaning with detergents, as follows:

(1) Solvent Degreasing.The
most commonly used solvent is trichloroethylene (commonly called trichloroethylene),
generally in multiphase operation. For example, three-phase operation involves
washing in the heated liquid, partial rinsing and cooling by dipping into cold
condensed liquid, and finally vapor rinsing by suspending the cooled die casting
in the heated vapor. Where the soil is packed very hard, mechanical action such
as power spraying or washing with the solvent must be included in the degreasing
cycle. A sequence involving immersion in vapor, washing in a pressure spray
of clean, condensed liquid, followed by a final vapor rinse, has received attention.

The trichloroethylene must be inhibited.
Finely divided zinc and aluminum catalyze the decomposition of this solvent
to produce harmful free acid. Frequent or continuous distillation is very important
to avoid acidity.

Trichloroethylene does not burn,
but its vapor is toxic. Degreasers are not ordinarily ventilated but are designed
with proper shielding against stray air currents to minimize both the health
hazard (Note) and loss of solvent.

Suppliers of trichloroethylene should
be consulted as to the safety of a given installation. Carbon tetrachloride
may also be employed for liquid solvent degreasing but since it is more toxic
than trichloroethylene its use is not recommended. Tetrachloroethylene has attracted
some attention but is not widely used at present.

In order to reduce solvent cost and
to minimize the toxicity hazard, some plants use mineral solvents such as high
test gasoline, kerosine, and mineral spirits. Like trichloroethylene, these
solvents require agitation to loosen packed buffing compound. They offer no
technological advantage over trichloroethylene, but they are cheaper. The fire
hazard is obvious.

Mineral solvents are not easily removed
during alkaline cleaning and tend to be dragged over into the plating solutions.
Soak cleaning or thorough washing after mineral solvent degreasing is a usual
requirement

NOTE Caution.Prolonged exposure
to atmospheres containing more than 200 ppm of trichloroethylene may cause head
aches and permanent body damage. Exposure to vapors in quantity may cause death.

(2) Emulsion Cleaning.Packed
buffing compound may be loosened and to some extent removed by immersion in
various emulsions. These frequently are composed of kerosine, emulsified with
pine oil, soap and water. Power spraying or washing or other means for creating
more or less violent contact with the work is required for best removal of packed-in
soil. A soak cleaner or thorough washing is required to insure that no kerosine
is carried over into the plating solutions. Inferior or poorly maintained rack
coatings and rough flaky deposits built up on the racks will increase the danger
of oil carry-over and must be avoided.

(3) Soak and Spray Cleaners.Soak
cleaners are usually built around suitable detergents which tend to soften and
loosen packed soil deposits. Frequently, they are used in conjunction with force
spraying or washing to add mechanical action to the softening effect of the
detergent. Since excessive contact with strong alkalies must be avoided in cleaning
zinc-base die castings, soak solutions should be as nearly neutral and the time
of soaking as short as possible. Some solutions are difficult to remove by rinsing,
so care should be taken in the selection to insure the best probability of adequate
cleaner film removal.

ELECTROCLEANING
8. (a) Generally speaking, it is not possible to insure completely adequate
cleaning in degreasing or precleaning operations. An electrolytic cleaning stage
is required. It is the function of the precleaning operation (or operations)
to remove all but traces of soil. If it does not do so, it is unlikely that
the electrocleaning operation will correct the deficiency; the result may be
a spotty final cleaning.

(b) A wide variety of proprietary
cleaners is available commercially. Some of these are designed specifically
for use with direct current (work as cathode) while others are intended for
use with reverse current (work as anode). Cleaners designed for one direction
of current flow should not be used with the other direction. While both types
of solution have been used successfully, the so-called anodic cleaners greatly
predominate today. Stains produced in anodic cleaners are more readily removed
in the acid dip than those from cathodic cleaners.

(c) Attention must again be called
to the need for minimizing alkali attack on zinc-base die castings. The entire
sequence of cleaning operations must be considered in this connection. An electrocleaner
which is safe for use after simple trichloroethylene degreasing may be too strong
when its alkalinity effect is added to that of a previously used alkaline soak
cleaner. Similarly, the relatively long immersion times required by some automatic
conveyors may make it necessary to reduce the cleaner concentration or its temperature
or perhaps to change to a milder cleaner. The better the precleaning the greater
is the danger of overcleaning in the electrocleaner.

(d) Rinsing.Rinsing after cleaning
must be thorough. Wherever feasible, hot rinsing followed by cold rinsing should
be used. Removal of solutions from holes and cavities may be facilitated by
the use of directed sprays. Good rinsing practice frequently includes both spray
and dip rinsing. Agitation of dip rinses is helpful. The efficacy of a rinse
will increase with temperature; very low temperatures should be avoided.

ACID DIP
9. (a) An acid dip must follow alkaline cleaning. There is good evidence that
the strength of the acid and the time of immersion should be adjusted to fit
the cleaning cycle. In general, dilute acid dips should be used after short
contacts with dilute mild alkalies and more concentrated acids after prolonged
contact with more strongly alkaline solutions. Dilute solutions of sulfuric
acid ( to 1 per cent by weight) are commonly used. Similar solutions of hydrochloric
acid and to some extent hydrofluoric acid are suitable. For very mild acid dipping,
citric acid has been suggested.

(b) Rinsing. Since cyanide copper
solutions frequently are used as the first plating step; thorough rinsing after
the acid dip is required.

COPPER STRIKE
10. Where copper is the first coating to be applied, it may be desirable to
use a cyanide copper strike before entering the main copper plating step. The
danger of copper blister formation may be minimized by such practice. The solutions
used by individual platers vary somewhat in composition and operating conditions
but generally fall within the range given below (the Rochelle salt addition
is commonly used):

Copper 1.8 to 3.5 oz per gal (13.26
g/l)
Free cyanide 0.75 to 1.5 oz per gal (5.6-11 g/l)
Rochelle salts 2 to 5 oz per gal (15-37.5 g/l)
Sodium carbonate 2 to max 8 oz per gal (15-60 g/l)
pH 11.5 to 12.5
Current density a, 15 to 75 amp per sq ft (1.6 to 8.1 amp/dm2)
Temperature 120 to 140° F (49 to 60° C)
Plating timeb 15 sec to 3 min
a Current should be on when work enters the solution.
b Varies with current density usedshortest time with highest current density.

Following the copper strike, the
plating may be continued in any normal cycle.

APPENDIX

EFFECT OF CLEANING CYCLE
Failure to achieve a proper cleaning cycle will result in separation of the
plate usually in the form of blisters. When all of the grease has not been removed
(undercleaning), the red copper surface will be seen on the back of detached
particles of the coating. When the part has been overcleaned, the back of the
coating will appear gray. This is due to the formation of white-gray alloys
by diffusion between the zinc and the copper plate and the separation of the
plate through these alloys. A gray film beneath the copper on detached blisters
is almost infallible evidence of overcleaning.

The information contained in this site is provided for your review and convenience. It is not intended to provide legal advice with respect to any federal, state, or local regulation.
You should consult with legal counsel and appropriate authorities before interpreting any regulations or undertaking any specific course of action.

Please note that many of the regulatory discussions on STERC refer to federal regulations. In many cases, states or local governments have promulgated relevant rules and standards
that are different and/or more stringent than the federal regulations. Therefore, to assure full compliance, you should investigate and comply with all applicable federal, state and local regulations.